This invention relates generally to steam generators used in pressurized water reactor (PWR) nuclear power plants, and more particularly to a system and method of controlling steam generator level throughout the entire range of reactor load without the need of a manual or automatic transfer between a low power controller and a high power controller. As used herein, the term "steam generator level" refers to the level of secondary loop feedwater contained in the steam generator.
The primary function of a process controller, of course, is to maintain one or more process variables at or near desirable set points. However, since the human operator is responsible for the performance of the process, some means must be provided to enable him to verify that the controller is doing its job and to enable him to take over the control of the process if necessary. A control station provides this link between the operator and the process.
There are five typical parts to every control station: (1) a process-variable indicator, (2) a set-point adjusting mechanism, (3) an adjustment device (usually called manual) that directly manipulates the signal to the control valve (4) an output signal indicator, and (5) a device for switching between automatic mode and manual mode control. The implementation of the five control-station essentials will vary among manufacturers, but they usually are present in any control loop whether it be pneumatic, electronic, or computerized.
Process-variable indicators appear in many forms. In large-case instruments, the indicator may be a pointer on a circular or a horizontal scale, and may be part of a controller, a recorder, or both. Current configurations, in general, separate recorders and controllers such that the control stations are joined with the controllers to become indicating controllers. For each configuration of recorder or indicator, there is a different way to manipulate the set-point. In pneumatic instruments, the set-point mechanism generally adjusts a small transmitter which supplies a three to 15 pounds per square inch signal to the controller. Electronic instruments on the other hand generally provide a means for adjustment which consists of a potentiometer generating a voltage of opposite polarity to the process-variable signal.
The adjustment device is usually a manually operated device which directly manipulates the output signal to the valve or valve actuator. It is used only when the automatic controller is not in service. Most manual outputs are directly proportional to the position of the adjusting device. However, in some of the newer controllers, manual adjustments change the output by actuating an "increase" or "decrease" mechanism. An output signal indicator is a device which indicates what signal level is being applied to the control valve. The types of indicators used for such purposes are similar to those used for process-variable indicators. Normally, such indicators will have the ends of the scale labeled as "open" or "closed," referring to the position of the control valve.
The purpose of the auto-manual, or bumpless transfer switch is to change the source of the signal to the valve. To do this without changing the signal size, that is, with bumpless transfer, older controllers require a balance procedure. When switching from the normal or automatic mode to manual, the transfer device is first moved to an intermediate or balanced position and the manual output is matched with the automatic controller output. The transfer lever is then moved to the manual position to complete the bumpless transfer, with the output being adjusted to the desired value. When going from manual to automatic, the balanced position is used to match the set-point with the process variable before switching to automatic. Then the set-point is adjusted to the desired value.
In currently available process-control stations, the balanced position is being removed, each manufacturer generally using a different scheme. The most common method is to force the manual-adjust output to track the controller output while in automatic mode and to make the set-point-adjustment output track the measurement while in manual. Thus when switching, no balancing or alignment is necessary.
The control of feedwater level in the secondary loop of a nuclear steam supply system, however, is exemplary of situations where control is difficult because the system behaves with non-minimum phase dynamics. Non-minimum dynamics is a term used to describe a property of the frequency domain transfer function between plant input function and plant output function. Transport lags or pure-time delay between an input signal and its corresponding output is one form of non-minimum phase behavior. Another form of non-minimum phase behavior is an initial negative response of an output signal before changing sign and approaching its positive asymptote. This type of non-minimum phase behavior or what is often called by operators of such nuclear steam supply systems "shrink/swell behavior," is usually associated with plants with transfer functions containing right half plane zeros.
Changes in reactor power, steam flow, feedwater temperature and feedwater flow all affect the measured level of secondary loop feedwater contained in the steam generator. The level controller's basic task is, therefore, to maintain level on target and within limits by changing feedwater flow to compensate for changes in level produced by the other factors. The main consequence of the long lags and shrink and swell effects is that a controller must anticipate the affects of changes in plant state or control actions on steam generator level, and make compensatory responses before the ultimate effect of the event on steam generator level is manifested in measured level. In addition, the controller must keep track of past control actions and changes in plant state in order to interpret current steam generator level behavior. As a result, the process cannot be controlled easily by simple feedback of the error level. If the feedwater controller waits for an effect to be manifested in terms of level error before taking compensatory actions, the system can become unstable. This is because, given the long lags in the system, responses made after a disturbance in level is seen are not likely to produce an effect on level in time to avoid crossing a limit. Shrink and swell effects complicate the situation further. Because of such effects, the control action that ultimately brings the system back into balance initially exacerbates the problem. For example, if the operator waits until he sees the level decreasing before adding water, the water he adds will initially cause further decrease in level due to shrink, making a limit crossing more likely.
In addition to the difficulties associated with the complex process dynamics, prediction is difficult because the controller often does not have direct access to the critical state variables such as the steam generator water mass inventory, but only relies on indirect measures, such as turbine power, steam flow, and steam generator level. Furthermore, most critically accurate measures of steam flow and feedwater flow are not available at low power. As a result, the controller has no direct way of knowing whether steam flow and feedwater flow are in balance or how much of a change in feedwater flow is required to bring them into balance. The operators instead are forced to rely almost exclusively on level trend data to infer such information. Because of the long lags and shrink and swell effects discussed herein above, there is a significant delay before information about the feedwater flow-steam flow balance is manifested in steam generator level behavior.
Conventional three-element controllers used to measure steam generator level, steam flow, and flow in PWR nuclear power plants are, accordingly, not effective at very low power levels two reasons: (1) the lack of accurate flow measurements, and (2) the change in steam generator transfer function occurring at low feedwater temperatures. As a result, in existing designs the steam generator level control is transferred either to manual control or to a different controller for operation in the low power range. It is imperative in both cases that the transfer be "bumpless" if a reactor trip on high/low steam generator level is to be avoided.